-LRB- CNN -RRB- -- Investigators expanded their search into Texas Monday for a 7-year-old girl who was reported missing after her mother was found dead in Oklahoma Sunday night .

Authorities also are trying to find the dead woman 's estranged husband , the Oklahoma State Bureau of Investigation said in a statement .

The girl -- Aja Daniell Johnson -- was reported missing from Geronimo , Oklahoma , late Sunday , the bureau said .

Her mother , Tonya Hobbs , 37 , was found dead inside a parked RV Sunday night , the bureau said . Hobbs and her daughter were visiting Hobbs ' estranged husband , Lester William Hobbs , at his sister 's home , the bureau said .

Lester Hobbs , 46 , and Aja are missing and are presumed to be in Hobbs ' car , said investigators who issued an Amber Alert to try to find her . Lester Hobbs is not Aja 's biological father , police said .

Texas authorities also issued an Amber Alert , Texas Department of Public Safety spokeswoman Lisa Block said . Authorities believe Hobbs has connections to Rockport , Texas , and the alert was issued preemptively , she said .

Tonya Hobbs and her daughter were last seen Saturday night . Authorities did not say how Hobbs died , but the bureau said investigators believe someone killed her .

The RV belonged to Lester Hobbs , and was parked at his relatives ' home , Richard Goss , agent in charge of the state bureau of investigation 's Lawton , Oklahoma , office , told reporters Monday . Hobbs ' relatives became suspicious Sunday after they did not see Tanya Hobbs or Aja , and one of them pried open the door and discovered Tanya Hobbs ' body , he said . Lester Hobbs and Aja were gone , Goss said .

The Hobbses were separated , but apparently Tanya Hobbs was visiting Lester Hobbs in an attempt to reconcile , Goss said . The couple had been together for about five years .

A medical examiner was performing an autopsy , the bureau said , and police want to question Lester Hobbs about the death .

He and the girl are believed to be traveling in his car , a white 1992 Toyota Paseo with Oklahoma tag number 577-BPW . The two-door car has no hubcaps , and the rear passenger window is covered in plastic , the bureau said .

The Comanche County , Oklahoma , Sheriff 's Office said Aja has a medical condition that requires medication .

`` Our main concern at this time is to find the little girl , '' Sheriff Kenny Stradley told reporters . `` We know that she does need some medication , '' and authorities were checking leads regarding that , he said . He did not specify Aja 's medical condition .

Goss said Lester Hobbs has an extensive criminal history but did not elaborate . The Sheriff 's Office said he had prior felony convictions .

Lester Hobbs was supposed to appear in court in Lincoln County , Oklahoma , on a DUI charge , Goss said , and did not appear , so a warrant has been issued for him .

Aja 's biological father was awarded emergency custody of her in November , according to Oklahoma County District Court documents obtained by CNN affiliate KWTV of Oklahoma City , Oklahoma . At a hearing , Tonya Hobbs -- identified as Tonya Dunkin in the documents -- and the girl 's father , John Johnson , agreed that she would have supervised visitation with Aja and keep the girl away from Lester Hobbs , the documents said .

Aja is 4 feet tall and weighs 59 pounds , Goss said . She has brown eyes , and her hair is brown with the ends dyed black .

Lester Hobbs is about 6-foot-1 and 190 pounds , with hazel eyes and brown hair , according to the Oklahoma State Bureau of Investigation . The agency released a picture of him and said he has a mustache but no longer has a goatee .

Geronimo is about 12 miles south of Lawton , Oklahoma , in the south-central part of the state .

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NEW : Amber Alert issued as authorities extend search into Texas

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7-year-old was reported missing after mother was found dead Sunday night

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Authorities seek mother 's estranged husband , say she , girl visited man at his sister 's home

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Man , girl believed to be traveling in a white 1992 Toyota Paseo